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FDP Forum / Amp Mods, Repairs, and Projects / Biasing the Vibro-King by ear

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RKSTRAT
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USA

"Clapton is Good"
Jul 31st, 2011 12:25 PM   Edit   Profile   Print Topic   Search Topic



This is a spin-off of my other thread, but I thought I my need someone with direct experience with this.

The Vibro-King manual says to bias by ear. The instructions are to set the volume on 1 1/2 to 2. And find the maximum hiss. Then strum the guitar and decrease the bias until warmth is lost.
Then, turn it up a bit.

My tech suggested that I try this, as it will be the most efficient and productive way to get the sound I like. He had been setting the bias for me with a meter, but it needs some tweaking to get my sound.

Here is my experience trying to bias by ear. My tech says that the pot has about 280 degrees of motion. I found that the hiss plateaued at about 1/3 from max. I backed off of that by a hair,
and the amp sounded great. I have left it there for now.

My mild concern is that the tubes may be running a little hot. My understanding is that the bias pot on the V-K is pretty narrow, to protect the tubes. This in not my main gigging amp, so the tubes are not going to get a lot of work.

Anyone have experience with V-K biasing by ear? What kind of bias pot settings have you found in terms of the pot play?

ECS-3
Contributing Member
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USA / Virginia

Jul 31st, 2011 12:39 PM   Edit   Profile   Print Topic   Search Topic

"The Vibro-King manual says to bias by ear"

I don't see that statement in the online Fender Vibro-King User Manual?

Vibro King User Manual

RKSTRAT
Contributing Member
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USA

"Clapton is Good"
Jul 31st, 2011 12:46 PM   Edit   Profile   Print Topic   Search Topic

Yes, you need to search for the "Vibro-King Service Manual".
You will find the PDF. Sorry for the confusion.

another link here

(This message was last edited by RKSTRAT at 12:47 PM, Jul 31st, 2011)

Fritz_D_Cat
Contributing Member
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better living

through chemistry!
Jul 31st, 2011 12:48 PM   Edit   Profile   Print Topic   Search Topic

Use a meter.

ECS-3
Contributing Member
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USA / Virginia

Jul 31st, 2011 12:52 PM   Edit   Profile   Print Topic   Search Topic

I just looked at the Vibro King schematics online at Fender. There are several revisions of the schematic but they all show a recommended bias voltage on the schematic. I don't see anything in the engineering data about biasing the amp by ear.

Where are you seeing this recommendation to bias the amp by ear?

RKSTRAT
Contributing Member
*******

USA

"Clapton is Good"
Jul 31st, 2011 12:59 PM   Edit   Profile   Print Topic   Search Topic

You need to Google "Vibro-King Service Manual".

Hammond101
Contributing Member

So. Cal. USA

Jul 31st, 2011 03:16 PM   Edit   Profile   Print Topic   Search Topic

Do as you wish but after put a meter on it and see where it is biased.

With a 475V on the plates you should come out no more than 44ma or 70% plate saturation. If you go above that you will shorten tube life and risk tube failure during a gig. I would think 38-40ma would be about perfect, closer to a 60% saturation. Reduce these numbers by 2ma for 500V on the plates.

I don't bias any amp by ear. My ear will tell me if an amp is set up too cold however. Plugged in there is a loss of "feel" when she it set up too cold. Harmonics and sustain suffer, break up is "edgey".

You might want to try a Browner or Amp Saver on it and bring the line voltage down to 110 VAC. Could make all the difference in the world in your tone.

The VK is what it is. If it is properly set up and it doesn't give you the tone you want it's time to go amp shopping.

Just my $0.02 US

RKSTRAT
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USA

"Clapton is Good"
Jul 31st, 2011 03:32 PM   Edit   Profile   Print Topic   Search Topic

It was sounding perfect. Biased a 36 mA, but then the reverb tube went wild, although I misdiagnosed it. So we played with the bias and power tubes. Now I have harder tubes, and low bias was set and measured, probably 32 mA as I re-call. After a talk with my tech, he recommended just biasing by ear, since this is not my main amp, so not much risk of burning up tubes.

Fritz_D_Cat
Contributing Member
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better living

through chemistry!
Jul 31st, 2011 03:50 PM   Edit   Profile   Print Topic   Search Topic

Bad advice. Go by numbers first, ear second.

RKSTRAT
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USA

"Clapton is Good"
Jul 31st, 2011 04:09 PM   Edit   Profile   Print Topic   Search Topic

The Vibro-King Service Manual states that metering is not necessary for proper biasing of the Vibro-King. It can be biased by ear. Anyone have experience with this?

I am not implying that this is the best way to bias any amp.
I would like to remind you that the V-K has an external bias pot,
so presumably it is meant for such use.



willie
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Lawrenceville, GA

"Sound Service Since Sixty-Two"
Jul 31st, 2011 04:34 PM   Edit   Profile   Print Topic   Search Topic

"The bias, measured at pin 5 of V7 or V8 should be set to
–46Vdc. However, the Bias need not be measured to be
accurately set. To set the Bias without removing the
chassis, set the Volume knob between 1-1/2 to 2. (The
Bias control is located on the under side of the chassis,
next to the power cable.) Adjust the Bias so the amp
produces the maximum Hiss. Strum a full chord and let
ring. Now, turn the Bias control back just to the point that
the sound of the guitar loses warmth and sustain.Then
increase it slightly. This will ensure minimum crossover
distortion and the best sound."

This from our service manual...although it says it, I wouldnt do it...use the meter.

willie
musicmenders.com

ECS-3
Contributing Member
******

USA / Virginia

Jul 31st, 2011 04:42 PM   Edit   Profile   Print Topic   Search Topic

OK I found the service manual.

Just like willie says its supposed to be set at -46VDC. The rest of that paragraph I interpret as a second choice, not the recommended procedure.

I would use the meter. That whole thing with listening is prone to getting it wrong if you ask me. You can unbutton the amp, use a meter, and button it up in less than 1 hour.


RKSTRAT
Contributing Member
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USA

"Clapton is Good"
Jul 31st, 2011 05:00 PM   Edit   Profile   Print Topic   Search Topic

Thanks everyone!

Yeah, I have taken it to my tech five times for biasing. Hauling the 70 lb. amp, and the expense, and the back and forth tweaking has gotten too much. As I said, we were troubleshooting, switching power tubes, re-biasing. I just set the bias by ear and it sounds great. If I just keep an eye out for red-plating, shouldn't it be okay?

willie
Contributing Member
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Lawrenceville, GA

"Sound Service Since Sixty-Two"
Jul 31st, 2011 06:35 PM   Edit   Profile   Print Topic   Search Topic

Yeah...what the hell..it's your amp and FMIC seems to think it's a swell idea, so go for it. Just keep a close eye on it. ;-)

willie
musicmenders.com

RKSTRAT
Contributing Member
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USA

"Clapton is Good"
Jul 31st, 2011 07:13 PM   Edit   Profile   Print Topic   Search Topic

I was hoping to get some experienced V-K bias-ers to describe the trim pot sweep and how far you turn it to get a hotter bias.
I am a perfectionist, and believe me, I was insistent on metered biasing.....until now.

(This message was last edited by RKSTRAT at 07:14 PM, Jul 31st, 2011)

pdf64

UK

Aug 1st, 2011 12:53 PM   Edit   Profile   Print Topic   Search Topic

You new opinion is not consistant with common sense. Power tubes don't red plate until maybe 200% of max plate dissipation.
A guy on other forums is recommending to adjust bias voltage until the plates just start to go red (in a darkened room!) then back off a little.
Your amp, do as you see fit.
Pete.

willie
Contributing Member
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Lawrenceville, GA

"Sound Service Since Sixty-Two"
Aug 1st, 2011 01:57 PM   Edit   Profile   Print Topic   Search Topic

Pete +1

RKSTRAT
Contributing Member
*******

USA

"Clapton is Good"
Aug 1st, 2011 03:14 PM   Edit   Profile   Print Topic   Search Topic

I am not sure what we are talking about anymore :-).

I just brought up biasing by ear as Fender mentions as an alternative method. I am tired of hauling it back and forth for every tweak.

I DON'T EXPECT OR WANT THE TUBES TO REDPLATE. I just mentioned that as a means to detect a major problem in the method. The instructions are kind of subjective, so I can imagine the bias being set too high if you are not careful.

As I see it, what can go wrong with this method?:

1. If bias is too low, the amp won't sound good, so you would turn up the bias until it sounds good.

2. If it is set too high, you will go through tubes too quickly or in the worst case the tubes will redplate.

I am not sure what you mean by "do as you see fit".
Maybe I am missing something, sorry.

(This message was last edited by RKSTRAT at 03:42 PM, Aug 1st, 2011)

ECS-3
Contributing Member
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USA / Virginia

Aug 1st, 2011 05:16 PM   Edit   Profile   Print Topic   Search Topic

At the risk of beating this thread to death:

Instead of hauling the amp around like you say you do, why don't you learn to simply remove the chassis, put a voltmeter on the correct test point and set the bias to the Fender recommendation?

Its easy to do and if you like to experiment its a good way to ensure the amp is in a good bias range and might even provide some additional satisfaction of knowing how to perform this procedure?

The problem with using your ears is that ears are not reliable instruments.

my two cents worth

RKSTRAT
Contributing Member
*******

USA

"Clapton is Good"
Aug 2nd, 2011 07:40 AM   Edit   Profile   Print Topic   Search Topic

I leave anything with live voltages for the pros.

The good news is that after talking with my tech yesterday he said that with the cold tubes in place (10s), the plate dissipation was low, even when he turned up the bias all the way. These were measurements he took a couple of weeks ago. So, he said that at least with the 10s I don't have to worry about pushing the tubes.

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FDP Forum / Amp Mods, Repairs, and Projects / Biasing the Vibro-King by ear




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